Microwave electronic packages are frequently produced from aluminum alloys due to low weight and good thermal dissipation. These packages are machined from thick aluminum or an aluminum alloy block. This block is relieved on one side to form a deep cavity within which an electronic circuit is placed. Small holes are formed in the package walls to accept feed-throughs and power connectors, respectively. A cover is placed over the cavity and attached by a suitable method. These packages are required to be hermetic from 10.sup.-5 to 10.sup.-8 helium cc/sec. maximum leak rate.
However, two of the major disadvantages of aluminum are high coefficient of thermal expansion and dewetting properties causing poor solderability. In order to be able to solder the aluminum, these microwave packages are typically electroplated with metals like nickel and/or gold. The feed-throughs and the power connectors which are fabricated from cold rolled steel, stainless steel and iron-nickel alloys are soldered into the holes and the windows along the side walls. There are a variety of solders used for this purpose by the industry.
The electronic signals are allowed to enter and exit the package via pins contained within the feed-throughs and power connectors. The feed-throughs contain a pin of desired metal surrounded by a bead of molten glass which is surrounded by a ring of cold rolled steel, stainless steel and/or iron-nickel alloy. The pin serves as an electrical connection to communicate with the electronic circuit inside the package. The glass provides electronic isolation between the pin and the package.
The reliability of the feed-through and the power connector attachment is typically very poor. Besides the difficulty of a good attachment during manufacture, these joints commonly fail upon thermal cycling. There are two recognized reasons. First, poor nickel and/or gold plating of the packages, feed-throughs and power connectors or excessive leaching of the plated metals during soldering. This results in exposure of dewetting aluminum surface which inhibits soldering. The second reason is mismatched expansion between the aluminum or aluminum alloy of the package and the feed-throughs and power connectors. The coefficient of thermal expansion of aluminum alloys is 22.times.10.sup.6 in/deg.C/in. vs. that of cold rolled steel and stainless steel at 12.times.10.sup.-6 and iron-nickel alloys at 7.times.10.sup.-6. This mismatch in expansion during thermal cycling creates stresses which causes loss of the hermeticity and expensive rework and repeat of testing. In frequent situations upon multiple recurrence, the package becomes useless and is discarded.
In a recent development, some package manufacturers have attempted to develop new glasses that are compatible to aluminum. This, if successful, may allow direct glass sealing of pins into aluminum side walls, allowing most of the foregoing problems to be solved. Development of these low temperature glasses, however, will impose certain process alterations that may or may not be acceptable.
Patents which are relevant to the present invention are:
Wilson U.S. Pat. No. 4,906,957 which discloses an electrical circuit interconnect system that employs an electrically conductive enclosure and cover which completely encompasses, hermetically seals, and electrically isolates from the outside environment a component mounted on a first surface of an insulating substrate of a microwave circuit. A plurality of conductors mounted on the first surface of the insulating substrate electrically connect the component to the outside electrical circuitry by passing through a corresponding plurality of pass-through bores within the base of the enclosure. Specifically, within each respective pass-through bore, a corresponding glass encased conductor electrically connects each conductor within the enclosure to a conductor outside of the enclosure.
Carnahan et al. U.S. Pat. No. 4,816,791 disclose a transition between stripline transmission lines that includes a coaxial section placed between pads at the ends of the stripline conductors. The coaxial section is formed by a resilient center conductor surrounded by an incomplete circle of pins connected to the ground planes and forming the outer conductor. The connections to the pads enter the ends of the coaxial section at the azimuth of the gap in the circle pins. Good high frequency performance, despite the discontinuity between the pads and coaxial center conductor, is achieved by increasing the characteristic impedance of the coaxial section and that of the stripline near the transition relative to the characteristic impedance of the stripline remote from the transition.
Owens U.S. Pat. No. 4,799,036 discloses a radio frequency coaxial transmission line vacuum feed-through that is based on the use of a half-wavelength annular dielectric pressure barrier disk, or multiple disks, comprising an effective half wavelength structure to eliminate reflections from the barrier surfaces. Gas-tight seals are formed about the outer and inner diameter surfaces of the barrier disk using a sealing technique which generates radial forces sufficient to form seals by forcing the conductor walls against the surfaces of the barrier disks in a manner which does not deform the radii of the inner and outer conductors, thereby preventing enhancement of the electric field at the barrier faces which limits voltage and power handling capabilities of a feed-through.
Bennett U.S. Pat. No. 4,642,578 discloses a radio frequency circuit for ICRF heating that includes a resonant push-pull circuit, a double ridged rectangular waveguide, and a coupling transition which joins the waveguide to the resonant circuit. The coupling transition includes two relatively flat rectangular conductors extending perpendicular to the longitudinal axes of a respective cylindrical conductor to which each flat conductor is attached intermediate the ends thereof. Conductive side covers and end covers are also provided for forming pockets in the waveguide into which the flat conductors extend when the waveguide is attached to a shielding enclosure surrounding the resonant circuit.
Baird et al. U.S. Pat. No. 4,487,999 disclose an all-metal microwave chip carrier with subminiature ceramic feed-throughs, each configured to function as a coaxial cable having a predetermined impedance. In one embodiment, the feed-throughs are formed by providing ceramic tubing metallized inside and out in which the ends are cut away to provide half-cylindrical bonding pads. In order to permit bonding directly to the feed-through, a flat wire lead is soldered to the channel in the ceramic tube, with the ends of the flat wire extending onto the flat portions of the half-cylindrical portions of the feed-through. In one embodiment, the chip carrier includes a base, ring and stepped lid, all made of Kovar or other suitable material, with the lid being weldable to the ring rather than being brazed or soldered.
Schafer et al. U.S. Pat. No. 4,486,726 disclose one end of a coaxial cable that is telescoped into one end of a microwave component such as an attenuator with the outer jacket of the cable being metallurgically bonded by solder to the metal housing of the component.